2.1.7 [XML] Section 3.3.2, Attribute Defaults
Validity constraint: Attribute Default Value Syntactically Correct The declared default value MUST meet the syntactic constraints of the declared attribute type. That is, the default value of an attribute: - of type IDREF or ENTITY must match the Name production; - of type IDREFS or ENTITIES must match the Names production; - of type NMTOKEN must match the Nmtoken production; - of type NMTOKENS must match the Nmtokens production; - of an enumerated type (either a NOTATION type or an enumeration) must match one of the enumerated values. Note that only the syntactic constraints of the type are required here; other constraints (e.g. that the value be the name of a declared unparsed entity, for an attribute of type ENTITY) will be reported by a validating parser only if an element without a specification for this attribute actually occurs.
MSXML3 and MSXML6
When an attribute definition has a declared default value
that has a reference to an undefined entity, but is never used in a document,
the expected result is to have no errors; the actual result is a
"Reference to undefined entity 'X'" error, where
X is the declared default value.